Cao et al.
By contrast, ionized sec-n-alkyl aryl ethers exhibit stereo-
chemical selectivity in their expulsions of neutral alkenes with
very little hydrogen scrambling.7 Many studies have shown that
the resulting ions have phenolic structures.7,8 In these com-
pounds, elimination occurs via 4-membered cyclic transition
states in preference to passing through ion-neutral complexes.
As eq 3 exemplifies, this elimination is not regiospecific.
Labeling studies reveal that ionized 2-n-alkyl phenyl ethers expel
a mixture of 1- and 2-alkenes.8
The rate coefficients in eq 3 correspond to the three isomeric,
neutral alkenes produced by vicinal elimination: k′ for expulsion
of the 1-alkene, kC for expulsion of the cis-2-alkene, and kT for
expulsion of the trans-2-alkene. A labeling strategy for eq 3,
which parallels that of eq 2, permits assessment of the trans vs
cis preference. If the contribution from k′ can be determined,
placing a deuterium at the methylene vicinal to the aryloxy
group, as in Scheme 1, allows kT/kC to be measured.
Consider eq 3, in which k′ is neglected. Scheme 1 portrays
two racemic diastereomers (called erythro and threo) that differ
only by configuration at a monodeuterated methylene. The
scheme summarizes outcomes expected for the staggered
conformations about the C2-C3 carbon-carbon bond. Syn-
elimination is stereospecific. The preference for one double bond
geometry over the other constitutes the stereoselectivity of the
reaction.
We report here that tuning the ionization energy (IE) of the
aryloxy group gives a regiospecific variant of eq 3, which
produces ionized alkene (as does eq 2). This regioselectivity
permits relative rate constants to be extracted from the data.
If product analysis were not complicated by the competing
expulsion of a 1-alkene (the uppermost pathway in eq 3,
corresponding to rate coefficient k′), then observation of different
ratios of H to D transfer between the erythro and threo isomers
would provide a direct quantitative measure of stereoselectivity.
Ionized 3 displays the requisite regioselectivity when it produces
ionized 2-hexenes. In seeking to evaluate stereoselectivity,
however, two other issues have to be confronted, as well.
Unfortunately, trans:cis ratios have not hitherto been available
for eliminations via 4-center transition states. The difficulty
arises because published experiments - e.g. the thermal expulsion
of HX from neutral sec-alkyl halides2 - have been done under
conditions where the geometric isomers of the products
equilibrate.
One way to look at 4-center eliminations is to examine
unimolecular dissociations of gaseous ions. Assaying stereo-
chemical preferences via mass spectrometry can suffer from the
complication that elimination may go in two directions - for
instance, the unimolecular eliminations in eq 2 give ionized
1-butenes in competition with the products shown, a contribution
that needs to be taken into account. For an experiment to provide
a more accurate quantitative measure, the elimination needs to
be regiospecific. Here we describe such a case.
Compounds such as tartrate or dibromosuccinate esters would
appear at first glance to offer promising systems for studies
without the necessity of isotopic labeling, but published experi-
ments demonstrate that alternative decomposition pathways
dominate. Although the meso and racemic compounds exhibit
significant differences, ionized tartrate esters exhibit very little
water loss, and ionized 2,3-dibromosuccinate esters show
virtually no elimination of HBr.5
This paper looks at cations that can undergo conformational
interconversion prior to vicinal 4-center elimination. Given that
these ions are produced at low pressures with a broad internal
energy distribution (and do not exchange energy with a thermal
bath), the usual suppositions that underlie the Curtin-Hammett
principle might not be warranted. Interpreting unimolecular
dissociations using first-order kinetics therefore requires justi-
fication, for which we provide a posteriori tests of consistency.
Background
Secondary n-alkyl aryl ethers provide the most promising
examples of stereoselective 4-center eliminations to date. Gas
phase dissociations of ionized alkoxybenzene derivatives have
offered fertile ground for mechanistic investigations over a
period of more than four decades. Primary alkyl phenyl ethers
dissociate via ion-neutral complexes when ionized, regardless
of whether they become positively charged by protonation or
by removal of an electron.6
The first issue concerns the validity of algebraic manipulations
based on fragment ion abundance ratios. The data analysis
depends on how the ratios vary over the time interval of the
experiment. Finding solutions to the differential equations is
comparatively simple if the reaction has run to completion or
if it obeys first-order kinetics (such as when ions are monoen-
ergetic8 or are activated by repeated collisions with a bath gas
where the collision rate is much faster than the unimolecular
decomposition9).
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